The present invention relates generally to pump control systems and, more particularly, to closed loop pump output control systems for variable displacement piston pumps.
The present invention is an improvement on the invention described in U.S. Pat. No. 4,494,911 to Davis entitled "Piston Pump Servo Control." The disclosure in U.S. Pat. No. 4,494,911, including all written descriptions and illustrations, is hereby incorporated by reference into the present disclosure. To avoid possible confusion, like reference numerals are used whenever possible.
As noted in the above-mentioned patent specification, variable displacement axial piston pumps typically have swash plates which may be adjusted so as to control the pump output flow. These swash plates are usually mounted to be rotatable about an axis and the angular position of the swash plate is proportional to the pump output level. To provide precise control of the pump output, the angle of the swash plate can be measured to produce feedback signals which may be compared to signals representing the desired pump output. A difference or "error" between these signals is used to produce a control signal for adjusting the swash plate position and, thus, the pump output. It should be noted that this system does not take into account speed variations in the prime mover speed.
As hydraulic systems continue to develop and become more sophisticated, the need for more elaborate and precise controls increases. Electronic and electro-hydraulic controls for pumps permit the use of microprocessors and similar signal processing means for hydraulic system control. One of the problems associated with these electronic systems is the availability of dependable, cost efficient transducers, especially flow transducers. Turbine-type flow meters are available, but are costly and have a limited range of effectiveness. Measuring the pressure drop across a fixed orifice is also relatively costly and inefficient.
A known system for controlling the output rate of a variable displacement pump is described in U.S. Pat. No. 4,395,199 to Izumi et al. In the Izumi patent, a plurality of variable displacement hydraulic pumps are powered by an internal combustion engine. An engine speed deviation is obtained by calculating a difference between a target engine speed set by the operator and the actual output speed of the engine. The engine speed deviation is converted by processing circuitry into a pump control coefficient which is functionally related to the deviation. This coefficient is multipled with an output variable (L.sub.1 or L.sub.2) which varies with external manipulation of the operation levers of the pumps. The resulting product is summed with a signal which represents the inclination angle of the pump swash plate, and the resulting sum is used to control that angle. This system is relatively complex and is dependant upon the accuracy with which the engine speed can be measured and the degree to which engine speed is actually representative of the true pump speed, as illustrated by Izumi in FIG. 5. Steps also must be taken to prevent short term variations in speed of the internal combustion engine from interferring with operation of the control loop.
Accordingly, there exists a need to provide a simple and dependable electronic control system for variable displacement pumps which has increased precision and stability and which can be implemented in a cost efficient manner. An object of the present invention is to provide such a system.
Another object of the present invention is to provide an electronic control system for variable displacement pumps which utilizes a direct measurement of pump cylinder block speed to calculate an error signal which is used to control the position of the displacement determinitive element.
A still further object of the present invention is to provide an electronic control system for variable displacement pumps which include compensation for volumetric efficiency.
Yet another object of the present invention is to provide an electronic control system for variable displacement pumps wherein the pump cylinder speed sensor is mounted within the pump housing.
These and other objects are attained in an electronic control system for variable displacement pumps which includes means for measuring the actual position of the displacement determinitive element and the actual rotational speed of the pump cylinder block, and for producing a signal which is representative of the actual output flow rate of the pump. This signal is compared with a signal which is representative of the desired flow rate and an error signal is produced to control the position of the displacement determinative element. The signal representatives of actual output flow rate may be compensated for decreasing volumetric efficiency. The means for producing a signal indicated of the actual position of the displacement determinative element is preferably a rotary potentiometer mounted within the pump housing and directly connected to the swash plate. The rotary potentiometer is mounted in the pump housing such that its rotational axis is along a rotational axis of the positionable swash plate.
The means for producing a signal indicative of the actual rotational speed of the pump cylinder block is preferably an optical sensor mounted in the pump housing adjacent to the rotatable cylinder block. The optical sensing device produces a frequency signal by sensing the passage of marks inscribed on the outer surface of the cylinder block. This signal is preferably buffered and converted to an analog voltage signal which is combined by an analog multiplier with an analog signal produced by the rotary potentiometer. The resulting product represents a direct measure of the actual output flow rate of the pump. The means for compensating for decreasing volumetric efficiency includes a pressure sensor at the pump outlet to measure the load pressure. The pressure signal is added to the desired rate signal. In an especially preferred embodiment of the invention, the means for comparing the desired and actual flow rate signals includes a failsafe circuit for forcing the displacement determinative element to zero stroke position when any of the sensor(s) fail.
The present invention provides a method for measuring the output rate of a variable displacement pump. The pump has a housing, a rotatable cylinder block within the housing having a plurality of pistons reciprocating in the housing, and a positional displacement determinative element. The method includes the steps of producing a signal which indicates the actual position of the displacement determinative element, and producing a signal indicative of the actual rotational speed of the rotatable cylinder block. These signals are combined to produce a signal indicative of the actual output flow rate.
Other objects, advantages and novel features of the present invention will become readily apparent when the following detailed description of the preferred embodiment is considered in conjunction with the attached drawings.